In the prior art, the brightness of a liquid crystal display (such as a computer terminal screen) has been adversely affected by absorptive elements in the optical path. For example, the front and rear polarizers in a conventional twisted nematic liquid crystal display assembly can absorb more than two thirds (2/3) of the overall luminance emitted by the backlight. Display brightness can be enhanced by ensuring that more light is transmitted, either by increasing the backlight intensity or reducing the amount of light lost in the absorptive elements.
Increasing the backlight intensity has the major drawbacks of higher power dissipation (leading to shortened battery life for portable equipment) and shortened backlight bulb life. Although power dissipation and battery size and life are ever present challenges, the prior art fails to consider a closer examination of the absorptive elements in backlit "direct view" liquid crystal display applications, i.e. computer screens or televisions, with the goal of reducing the absorption. Conventional polarizers are made from stretched polyvinyl alcohol films containing iodine (or a dye if color is desired) between layers of cellulose acetate, attached to a glass surface with an acrylic adhesive and covered by a layer of plastic. Transmission does not exceed 40%, efficiency is lost over time, and polarizers are an expensive component in the manufacture of LCDs. According to the present invention, in a direct view application (such as a computer or television screen), the conventional rear polarizers are replaced with a high efficiency, non-absorptive chiral nematic liquid crystal polarizer whose transmissiveness is substantially greater than the absorptive polarizers of the prior art.
A. Liquid Crystal (LC) Polarizers
The concept of liquid crystal devices (LCDs) as display elements is familiar. The effect of electrical current through an LC display element backlit with polarized light produces the well-known "black line segments" which are featured in everything from digital watch displays to laptop computer screen text. By contrast, the application of a liquid crystal as a polarizer is not at all familiar or common. Before discussing the scant prior art which discusses liquid crystal polarizers, an overview of the characteristics of cholesteric liquid crystals (CLCs) is necessary to provide a basis for illustrating the present invention.
B. Cholesteric Liquid Crystal (CLC) Polarizers
Cholesteric liquid crystals (CLCs) are a class of liquid crystals exhibiting unique optical properties. Cholesteric liquid crystals were originally so named because the substances in which the pattern of molecular ordering was observed were related to cholesterol, hence "cholesteric". The more descriptive terminology for this class of liquid crystal is "chiral nematic". "Chiral", meaning "twisted", and "nematic", meaning "thread", succinctly express the gross appearance of the molecular orientations: like twisted threads. Popular and technical literature still frequently refer to the class by its early name. For the purposes of this discussion, however, the terms "cholesteric" or "chiral nematic" liquid crystal or "CLC" or "CNLC" will be used interchangeably, with the intention that each term be understood to include the entire class of chiral nematic liquid crystals.
The distinct helical molecular ordering imparts CLCs with several notable optical properties. First, CLCs are virtually non-absorptive. Light hitting a CLC is either transmitted or reflected. Second, CLCs exhibit circular dichroism for certain wavelengths of light. By way of explanation, consider the concept of polarization. While it is familiar to most to consider light as being either horizontally or vertically (linearly) polarized, it is useful and viable to alternatively conceptualize light as composed of two possible circular polarizations--right and left. Light can thus be conceived of as composed of righthanded and lefthanded circular polarizations. A chiral nematic liquid crystal allows wavelengths of light that are significantly longer or shorter than the mean refractive index of the liquid crystal multiplied by its pitch (the distance in which the liquid crystal molecular helical structure completes one complete twist) to simply pass through the liquid crystal. For wavelengths of light that are at or near the product of refractive index and the pitch, however, the interaction of the chiral nematic liquid crystal structure and the light's circular polarization is much like twisting a nut onto a screw. Light of the same polarization sense or handedness passes easily, like a right handed nut easily screws onto a right handed screw. However, light of the opposite polarization is reflected. This phenomenon of "selective reflection" as well as the characteristic non-absorption accounts for the virtual fifty percent transmission of incident light with a wavelength equal to the CLC pitch multiplied by the material refractive index.
This effectively optimal percentage of transmitted circularly polarized light is a vast improvement to the ultimate brightness of the active matrix liquid crystal display since it is only polarized light which eventually contributes to the screen images displayed to the viewer.
The band of light wavelengths subject to circular dichroism is a characteristic of the material used in the CLC polarizer, and is referred to as the device's "notch" or "notch bandwidth". Light wavelengths within the "notch" region will be circularly polarized by the CLC device and either transmitted or reflected. Light outside the "notch" is transmitted by the CLC. Only light in the "notch" region contributes to the display that is seen by the viewer of a direct view device, and therefore it is important that the "notch" region be broad enough to cover the full spectral range of the LCD backlight.
In short, the instant invention effectively eliminates the absorption problem heretofore inherent in conventional polarizers since the CLC does not absorb any appreciable amount of light. On an initial pass, nearly 50% of light in the notch bandwidth is circularly polarized on passing through the CLC polarizer, passes through a quarter wave plate where it becomes linearly polarized, and, ultimately, "feeds" the display.
C. Notch, Bandwidth and Birefringence
As it is apparent from the above discussion, the notch bandwidth of a CLC determines how broad a band of light wavelengths the CLCs will polarize. The notch or notch central wavelength (.lambda.o) is related to the device properties by the equation EQU .lambda..sub.o =n.times.p.times.cos.crclbar. (1)
where "n" is the average refractive index of the liquid crystal material, "p" is the helical pitch of the chiral nematic material, and "e" is the angle of the incident light. For normally incident light, the equation reduces to EQU .lambda..sub.o =n.times.p (2)
The notch central wavelength (.lambda..sub.o) is directly related to the liquid crystal's pitch (p).
The polarization (notch) bandwidth can be approximately expressed as EQU .DELTA..lambda.=.DELTA.n.times.p (3)
where ".DELTA.n" is the birefringence of the material. From this equation it can be seen that polarization bandwidth (.DELTA..lambda.) is directly related to material birefringence (.DELTA.n). Chiral nematic liquid crystal materials can be designed for high birefringence (.DELTA.n) and correspondingly broad bandwidth (.DELTA..lambda.). Since the CLC device is highly transmissive outside its "notch", multiple CLC devices (with different .lambda..sub.o 's) can be stacked to provide a broadband polarizer. However, a single broad notch bandwidth reduces the number of discrete components needed for broadband optical systems. The invention herein teaches a liquid crystal film or films of sufficiently broad bandwidth so as to polarize and transmit light of the entire visible spectrum (450 nm to 650 nm). As discussed below, it is the polymer chiral nematic liquid crystal which exhibits the optical and physical properties necessary for the invention taught herein.
1. Chiral Nematic LCs: Monomers Versus Polymers
Monomer CLCs used in a variety of applications differ markedly from a polymer chiral nematic LC used as a polarizer, largely due to the differences arising from monomer versus polymer structure.
Monomer LCs are made up of short, single chain molecules. Polymer LCs are made up of long chain molecules consisting of a sequence of repeating monomers which are connected by chemical bonds. In the polymers of the invention, the properties are derived from monomer LCs attached to a polymer backbone.
The polymer's physical and optical properties are quite different from those of the ubiquitous monomer. Because the optical properties of monomer LCs are typically defined in narrow spectral ranges, monomers are brilliant (due to the high transmissiveness/low absorption) but monochromatic (due to the narrow notch bandwidth). Polymer chiral nematic LCs can be designed with broad optical properties--making them more achromatic and, therefore, adaptable to full color displays. So too, monomers are typically extremely temperature sensitive and the related optical properties are temperature dependent (hence the popular use in thermometers). Polymer liquid crystals, however, have excellent thermal stability over a wide range of temperatures. Moreover, polymers also demonstrate environmental durability owing to a "frozen" mesophase at temperatures below 150 degrees C. Polymer chiral nematic liquid crystals also exhibit stable optical properties over a wide temperature range.
Another major difference between monomers and polymers is that, owing to the different physical properties, polymer chiral nematic liquid crystals can be fabricated as free-standing films or can be spread on a single glass substrate layer. Monomers, on the other hand, are found only in a relatively low viscosity (liquid) state, and therefore must be confined in a glass "cell", in which two sheets of glass contain the liquid crystal. The cell is sealed around the entire perimeter to keep the liquid crystal from leaking out.
A free-standing polymer chiral nematic liquid crystal film (which may be imagined as a plastic-like sheet) is dramatically lighter than the conventional double glass cell required for monomer liquid crystals. Alternatively, and in the interests of greater durability, a single glass substrate can be used to support a thin polymer chiral nematic liquid crystal film. As the present invention teaches, even the use of a single glass substrate can achieve a significant weight savings over either a conventional or monomeric polarizer.
2. Monomer Projection Polarizers
While polymer chiral nematic liquid crystals ("CNLC") are discussed in the literature, they have not been associated with display applications. Recent literature discusses application of monomer chiral nematic liquid crystal devices in a projection system such as might be suitable for high definition television. (Schadt, Martin and Funfschilling, Jurg, "Novel Polarized Liquid-Crystal Color Projection and New TN-LCD Operating Modes" SID 90 DIGEST p 324-6). Schadt and Funfschilling describe a system which uses three narrow bandwidth monomer LC polarizers tuned to red, green, and blue, respectively, to generate the projection display colors from the white display backlight. Id.
The current invention deals not with projection, but with a direct view device, and employs a polarizer composed of broadband polymer chiral nematic liquid crystal devices, as opposed to monomer liquid crystals.
Hence, the invention herein disclosed is remarkably different from any device to date. Monomer liquid crystal devices typically have narrow polarization (notch) bandwidths (usually less than 20 nm). The system described by Schadt (supra) uses three narrow bandwidth chiral nematic liquid crystal polarizers, tuned to red, green and blue, to generate the projection display colors from the white display backlight.
The invention herein discloses a device utilizing one or more polymer liquid crystal films, each of which has a broad bandwidth and, consequently, properly arranged, can create a film which transmits the entire visible spectrum (450nm to 650nm).
Moreover, the disclosed direct view application differs significantly from the prior art projection devices. The projection device used by Schadt was, arguably, suitable for viewing because the light through the monomer chiral nematic liquid crystal polarizer is collimated (i.e. in parallel beams). This had the practical effect of producing a projected image which appeared fairly stable to the viewer.
A monomer liquid crystal is unacceptable as a rear polarizer for a direct view display, such as a computer terminal. Because the polarization "notch" is a function of viewing angle, in a narrowband monomer LC polarizer for direct view, the polarizer notch(es) would appear to shift at the slightest position change of the viewer and, consequently, the polarization efficiency at the wavelength emitted by the backlight would suffer and the contrast of the computer screen would be markedly reduced, depending upon the viewer's position. Two persons watching the same computer game, for instance, would each see different image quality since they each experience different viewing angles. This so limits the usefulness of a direct view color display that monomers are not candidates for polarizers in a LC direct view display.
The polymer liquid crystal disclosed herein is free of this serious shortcoming because it has a broader bandwidth and produces an achromatic display that does not shift in color as the viewer changes viewing angle. Thus, two persons playing a computer game on an assembly employing the herein disclosed invention would each see similar image quality.